The present invention relates to a magnetic memory having magnetoresistive effect elements as memory cells.
Magnetic random-access memories (MRAMs) have magnetoresistive effect elements, exhibiting magnetoresistive effect, as memory cells for data storage. MRAMs have attracted attention as a next-generation memory for high-speed operability, large volume and non-volatility.
A magnetoresistive effect is a phenomenon in which the electrical resistance of a ferromagnetic material will vary while magnetic fields are being applied thereto, which depends on the orientation of magnetization in the ferromagnetic material.
MRAMs use this phenomenon for data storage in which data are stored as the orientation of magnetization in a ferromagnetic material and retrieved as change in electrical resistance of the ferromagnetic material.
A recent advanced ferromagnetic tunnel-junction structure having an insulating (tunnel-barrier) layer between two ferromagnetic layers exhibits 20% or more of magnetoresistance ratio (MR ratio) because of a tunnel magnetoresistive (TMR) effect (J. Appl. Phys., 79, 4724 in 1996). This advancement is the trigger for expectation and remark of MRAMs using a ferromagnetic tunnel-junction element based on the TMR effect.
In use of a TMR element for MRAMs, a magnetization-fixed layer, one of the two ferromagnetic layers, having a tunnel-barrier layer therebetween, in which magnetization is fixed, is used as a magnetization-reference layer whereas a magnetization-free layer, the other layer, in which the direction of magnetization can be easily inversed, is used as a storage layer.
Parallel magnetization and antiparallel magnetization between the magnetization-fixed and -free layers can be stored as data in the form of binary data “0” and “1”, respectively.
Data is written (stored) with inversed magnetization direction in the storage layer by means of magnetic fields induced by currents flowing wirings for writing provided near the TMR element. The written data is retrieved by detection of change in resistance based on the TMR effect.
The magnetization direction in the magnetization-reference layer is fixed by exchange coupling generated between the ferromagnetic layer and an antiferromagnetic layer provided as touching the former layer so that inversion of the magnetization direction rarely occurs. This structure is called a spin-valve structure.
The magnetization direction in the magnetization-reference layer in this structure is fixed by annealing with application of magnetic fields (magnetization-fixing annealing).
The direction of easy axis of magnetization in the storage layer is affected by given magnetic anisotropy so that it is almost the same direction as in the magnetization-reference layer.
Current-induced magnetic fields cause magnetic rotation in the storage layer, as discussed. It is preferable that the magnetic fields required for inversion of the magnetization direction in the storage layer is small. Easy occurrence of the inversion of magnetization direction, however, could cause malfunctions due to external noise magnetic fields or leak magnetic fields generated during writing in a memory cell adjacent to a target memory cell.
It is thus preferable for the storage layer that the inversion of the magnetization direction easily occurs not in a data-holding state but only in a data-writing state.
Small magnetic fields for the inversion of the magnetization direction is achieved with a soft magnetic material exhibiting small coercivity for the storage layer or a thin storage layer also exhibiting small coercivity.
A stable data-holding state is achieved with high shape anisotropy which is given by high ratio of long to short sides in storage cells of a TMR element.
The smallness in memory cells for high storage density is a preferable choice for high storage capacity in MRAMs. The shorter the short side (called cell width) and also the long side (called cell length) under the design rule, the more feasible for storage density.
Such a small cell structure, however, could have an aspect ratio (a ratio of cell length to cell width) of almost 1, which results in low shape magnetic anisotropy and hence very unstable magnetization in a data-holding state.